When we do the normal single crochet, it works on the top of a stitch. For the front post single crochet and the back post single crochet, it works on the post below the stitch. The pictures below are going to show you how to make the front & back post single crochet:
Front Post Single Crochet
Back Post Single Crochet
Step 3: Yarn over and pull yarn through the post
Step 4: Yarn through the loops and form a single crochet